HCMV is an ubiquitous virus that is present in over 60% of the population depending on socioeconomic status. Following primary infection, HCMV persists for the life span of the host. Although HCMV is generally benign in healthy individuals, the virus may cause devastating disease in immunocompromised populations resulting in high morbidity and mortality (for review, see (Pass, R. F. 2001. Cytomegalovirus, p. 2675-2705. In P. M. H. David M. Knipe, Diane E. Griffin, Robert A. Lamb Malcolm A. Martin, Bernard Roizman and Stephen E. Straus (ed.), Fields Virology, 4th ed. Lippincott Williams & Wilkins, Philadelphia)). Recent increases in the number of patients undergoing immunosuppressive therapy following solid organ (SOT) or allogeneic hematopoietic cell transplantation (HCT), as well as the expanded use of HCT for diseases such as sickle cell anemia, multiple sclerosis and solid cancers have increased the number of patient populations susceptible to HCMV disease (Chou, S. 1999. Transpl Infect Dis 1:105-14, Nichols, W. G., and M. Boeckh. 2000. J Clin Virol 16:25-40 and Sepkowitz, K. A. 2002. Clin Infect Dis 34:1098-107). HCMV is also the most common congenital viral infection, and the leading infectious cause of central nervous system maldevelopment in neonates (Fowler, K. B. et al. 1997. J Pediatr 130:624-30, Larke, R. P. et al. 1980. J Infect Dis 142:647-53 and Peckham, C. S. et al. 1983. Lancet 1:1352-5). In this regard, HCMV is considered the major cause of sensorineural deafness in neonates independent of infectious status (Fowler, K. B. et al. 1997. J Pediatr 130:624-30). HCMV therefore remains a major cause of mortality in multiple patient populations emphasizing the need for new antiviral pharmacologic and vaccine strategies. Immunity induced by natural wild-type (WT) CMV infection has consistently been shown unable to prevent CMV re-infection (see below). This unique characteristic of CMV presumably explains the poor efficacy of candidate vaccines in trials to prevent CMV infection (Pass, R. F. et al. 2009. N Engl J Med 360:1191-9). Nevertheless, immunity to HCMV acquired through natural infection has been shown to significantly decrease maternal to fetal transmission of HCMV during pregnancy. This observation would indicate that induction of an immunity in pregnant women that is comparable to that induced by natural CMV infection, but that is induced in a safe manner, may be able to decrease maternal to fetal transmission and have a significant impact on clinical CMV disease in the neonate. HCMV-specific T cell immunity has also been shown to afford protection against CMV disease in transplant patients, presenting another population wherein the ability to safely induce an immunity comparable to that acquired by natural CMV infection would have a clinical impact on CMV disease (Leen, A. M., and H. E. Heslop. 2008. Br J Haematol 143:169-79, Riddell, S. R., and P. D. Greenberg. 2000. J Antimicrob Chemother 45 Suppl T3:35-43 and Riddell, S. R. et al. 1994. Bone Marrow Transplantation 14:78-84). Cytomegalovirus is highly immunogenic, but has evolved immune evasion mechanisms to enable virus persistence and re-infection of the sero-positive host:
The immunological resources specifically devoted to controlling HCMV infection are enormous, with CMV being one of the most immunogenic viruses known. High antibody titers are directed against the main HCMV envelope glycoprotein (gB) during primary infection of healthy individuals (Alberola, J. et al. 2000. J Clin Virol 16:113-22 and Rasmussen, L. et al., 1991. J Infect Dis 164:835-42), and against multiple viral proteins (both structural and non-structural) during MCMV infection of mice (Farrell, H. E., and G. R. Shellam. 1989. J Gen Virol 70 (Pt 10):2573-86). A large proportion of the host T cell repertoire is also directed against CMV antigens, with 5-10 fold higher median CD4+ T cell response frequencies to HCMV than to acute viruses (measles, mumps, influenza, adenovirus) or even other persistent viruses such as herpes simplex and varicella-zoster viruses (Sylwester, A. W. et al. 2005. J Exp Med 202:673-85). A high frequency of CD8+ responses to defined HCMV epitopes or proteins is also commonly observed (Gillespie, G. M. et al. 2000. J Virol 74:8140-50, Kern, F. et al. 2002. J Infect Dis 185:1709-16, Kern, F. et al. 1999. Eur J Immunol 29:2908-15, Kern, F. et al. 1999. J Virol 73:8179-84 and Sylwester, A. W. et al. 2005. J Exp Med 202:673-85). In a large-scale human study quantifying CD4+ and CD8+ T cell responses to the entire HCMV genome, the mean frequencies of CMV-specific CD4+ and CD8+ T cells exceeded 10% of the memory population for both subsets (Sylwester, A. W. et al. 2005. J Exp Med 202:673-85). In this study, it was not unusual for CMV-specific T cells to account for >25% of the memory T cell repertoire of a specific individual or at specific tissue sites. The clinical importance of this high level of CMV-specific immunity is most clearly shown by the occurrence of multi-organ CMV disease in immune-suppressed individuals during transplantation, and the ability of adoptive transfer of T cells to protect these patients from CMV disease (Riddell, S. R. et al. 1994. Bone Marrow Transplantation 14:78-84).
Paradoxically, the high levels of CMV-specific immunity are unable to either eradicate the virus from the healthy infected individual, or confer protection of the CMV sero-positive individual against re-infection. This ability of CMV to escape eradication by the immune system, and to re-infect the sero-positive host has long been believed to be linked to the multiple viral immunomodulators encoded by the virus (for review, see (Mocarski, E. S., Jr. 2002. Trends Microbiol 10:332-9)). The HCMV US6 family of proteins (RhCMV homologues: Rh182-Rh189) are the most extensively studied of these immunomodulators (Loenen, W. A. et al. 2001. Semin Immunol 13:41-9). At least four different genes, US2, US3, US6 and US11—and the respective RhCMV homologues (Rh182, Rh184, Rh185, and Rh189)—are known to interfere with assembly and transport of MHC I molecules (Ahn, K. et al. 1996. Proc Natl Acad Sci USA 93:10990-5, Ahn, K. et al. 1997. Immunity 6:613-21, Jones, T. R. et al. 1995. J Virol 69:4830-41, Pande, N. T. et al. 2005. J Virol 79:5786-98, Wiertz, E. J. et al. 1996. Cell 84:769-79 and Wiertz, E. J. et al. 1996. Nature 384:432-8). Each of these four molecules interferes at different essential points of MHC I protein maturation. Briefly, US2 binds to newly synthesized heavy chain (HC) and reverse translocates the protein through the translocation channel SEC61 back into the cytosol where HC is degraded by the proteasome (Wiertz, E. J. et al. 1996. Cell 84:769-79 and Wiertz, E. J. et al. 1996. Nature 384:432-8). Similarly, US11 ejects MHC I back out into the cytoplasm (Wiertz, E. J. et al. 1996. Cell 84:769-). US3 and US6 act later in the MHC-I assembly process (Ahn, K. et al. 1996. Proc Natl Acad Sci USA 93:10990-5 and Ahn, K. et al. 1997. Immunity 6:613-21), with US3 retaining fully formed heterotrimers in the ER thus preventing their transport to the cell surface (Ahn, K. et al. 1996. Proc Natl Acad Sci USA 93:10990-5 and Jones, T. R. et al. 1996. PNAS USA 93:11327-33), and US6 preventing peptide transport by TAP (and thus formation of the trimeric complex of HC, β2m and peptide) (Ahn, K. et al. 1997. Immunity 6:613-21, Hengel, H. et al. 1997. Immunity 6:623-32 and Lehner, P. J. et al. 1997. Proc Natl Acad Sci USA 94:6904-9).
Consistent with persistent replication/chronic reactivation within the host, CMV also induces and maintains a characteristic and unique T cell immune response. Memory T cells induced by vaccination or infection may be broadly characterized into either effector (TEM) or central (TCM) memory, which follow from the distinct functions of these two memory populations (Cheroutre, H., and L. Madakamutil. 2005. Cell Mol Life Sci 62:2853-66, Mackay, C. R. et al. 1990. J Exp Med 171:801-17, Masopust, D. et al. 2001. Science 291:2413-7, Sallusto, F. et al. 1999. Nature 401:708-12 and Wherry, E. J. et al. 2003. Nat Immunol 4:225-34). TEM are designed for immediate function against the invading pathogen, being highly enriched at epithelial mucosal surfaces, are polyfunctional expressing high levels of multiple effector cytokines (expressing TNFα, IFNγ, MIP-1β effector molecules), and have high cytotoxic potential (CD8+). TEM and TCM may also be easily distinguished on the basis of cell surface markers, with TEM being CCR7−, CD28+/− and TCM being CCR7+, CD28+. Multiple studies indicate that persistently replicating viruses such as CMV maintain a T cell response that is heavily biased toward the TEM phenotype (Amyes, E. et al. 2003. J Exp Med 198:903-11, Appay, V., and S. Rowland-Jones. 2002. J Immunol Methods 268:9, Champagne, P. et al. 2001. Nature 410:106-11, Halwani, R. et al. 2006. Springer Semin Immunopathol 28:197-208 and Robinson, H. L., and R. R. Amara. 2005. Nat Med 11:S25-32). Indeed, CMV is regarded as the prototypic inducer of long-term TEM (Halwani, R. et al. 2006. Springer Semin Immunopathol 28:197-208, Holtappels, R. et al. 2000. Journal of Virology 74:11495-503, Robinson, H. L., and R. R. Amara. 2005. Nat Med 11:S25-32 and Sierro, S. et al. 2005. Eur J Immunol 35:1113-23). In contrast, analysis of T cell responses against non-persistent viruses (ie., influenza virus) in non-acutely infected humans, or following immunization with live non-persistent virus-based vaccines (YFV-17D, yellow fever vaccine, or Dryvax smallpox vaccine) shows that following a short-lived effector T cell phenotype, long-term virus-specific memory T cells against these non-persistent viruses is maintained primarily as TCM (Lucas, M. et al. 2004. J Virol 78:7284-7 and Miller, J. D. et al. 2008. Immunity 28:710-22).
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